Question 81852
Remember Pythagoras!
When you draw the diagonal (the line joining opposite corners) of a square, you divide the square into two congruent right triangles, right?
The diagonal is the hypotenuse (we'll call this side c) while the equal sides of the square are the legs of the right triangle (we'll call these sides a and b and, because they are equal, it doesn't matter which one is a and which one is b).
To quote Pythagoras...:"{{{c^2 = a^2 + b^2}}}" This is known as the Pythagorean theorem.
You know that the sides of the square (a and b) are each 20 feet long.  The Pythagorean theorem allows you to find the length of side c (the hypotenuse).

{{{c^2 = a^2 + b^2}}}  Substitute a = 20 and b = 20.
{{{c^2 = 20^2 + 20^2}}}
{{{c^2 = 400 + 400}}}
{{{c^2 = 800}}} Now take the square root of both sides.
{{{c = sqrt(800)}}}
{{{c = sqrt(400*2)}}}
{{{c = sqrt(400)sqrt(2)}}} Take the square root of 400.
{{{c = 20sqrt(2)}}}feet. This is the exact answer.

The approximate answer is:
{{{c = 28.3}}}feet. (to the nearest tenth)